In wireless communication systems, an air interface is used for the exchange of information between a mobile (e.g., cell phone) and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels and transmission over the communication channels can be carried out according to several well-known multiple access methods. Two such multiple access methods in wireless communications are time division multiple access (TDMA) and code division multiple access (CDMA).
As is well known, CDMA-based systems employ unique codes for multiplexing separate, simultaneous transmissions over a communication channel. In a CDMA-based system, transmission duration is fixed while the users share the resources in the code domain. In some systems, different resources can be allocated to different users by sharing the available code space unequally. A code-multiplexed CDMA system has several drawbacks relating to delays and memory requirements. For example, a larger fixed frame duration is generally required in a CDMA system, which leads to larger feedback delays. Additional memory is also required because the peak data rate needs to be supported over larger frame durations.
As is well known, TDMA-based systems transmit multiple digital transmissions in a communication channel by dividing the communication channel into multiple time slots so that multiple users can share the resources in the time domain. For example, a single user would be able to use all the available resources in a given time slot. By way of example, the resources include power, Walsh codes, antennas, and so on. In the well-known 1x-EV-DV standard, for example, the available resources are shared between circuit switched voice and data users within a standard 1.25 MHz channel bandwidth. Due to the real-time nature of the voice traffic, the resources (e.g., power, Walsh codes, etc.) are first allocated to voice. After satisfying the needs of real-time services, the remaining resources are then shared among the data users in a time-multiplexed fashion.
In TDMA-based systems, transmission duration can be variable in that a different amount of transmission resources can be allocated at different times. While a time-multiplexed TDMA system with variable frame duration may solve some of the aforementioned problems of code-multiplexed CDMA systems, there are other inherent drawbacks to a time-multiplexed system. For example, there is typically bandwidth inefficiency when transmitting shorter packets because the shortest frame duration cannot be smaller than one full time slot. Because all resources are allocated to a single user within a particular frame duration, resources are inefficiently used when a user only has a small amount of data to send due to padding or use of sub-optimal modulation and coding schemes. Moreover, data rate granularity is coarse because frame duration can only be multiples of one slot.